This application claims priority under 35 USC 119 of Japanese Patent Application No. 2000-8580 filed on Jan. 18, 2000, the entire disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a voltage applying means for a contact-transfer member that transfers a toner image formed on a photosensitive body of an image forming apparatus onto a recording medium. The present invention also relates to an image forming apparatus equipped with a switching means to apply transfer voltage after a cleaning sequence, and an image forming method employing said apparatus.
2. Description of the Related Art
In a conventional image forming apparatus employing an electrophotographic system, especially in a segment of the apparatus where a toner image is transferred to a recording sheet, an electrostatic latent image is formed by an exposing unit on a photosensitive drum, and visualized into a toner image using a toner supplied from a developing unit. Subsequently, this toner image is transferred to a recording sheet. The recording sheet bearing the toner image is passed through a fixing unit so that the toner image is fixed to become a copy. In the image transfer section where the image is transferred from said photosensitive body, a discharging member such as a corotron, and a contact-transfer roller are used. In a system employing said contact-transfer roller, the transfer member directly touches the photosensitive body at a position where there is a gap between any two successively fed recording sheets, resulting in a contamination problem that the toner adhered to the photosensitive body or paper particles attach to the contact-transfer member. Therefore, a cleaning sequence is programmed in a control unit such that the cleaning sequence is executed after a certain number of copies are made. In the cleaning sequence, generally voltage is applied to the transfer roller in opposite polarity to the transfer voltage that is applied for transferring procedure.
However, for example, if the transfer voltage, which is applied to the contact-transfer means, is +3 kV, and the voltage used during the cleaning sequence is xe2x88x92900V, the voltage gap is so big when the transfer voltage is applied after a cleaning sequence that there will be some lack of uniformity in potential on the surface of the photosensitive body. The lack of uniformity generated on the surface of the photosensitive body could lead to deterioration in the quality of recorded image. If the changes in the transfer voltage is very big and too frequent in a short period of time, it could lead to the deterioration of the photosensitive body, thus influencing the life of the image forming apparatus itself.
It is an object of the present invention to solve the problem of the voltage gap when the contact-transfer is given the transfer voltage, and in particular to eliminate the influence that application of the transfer voltage after the cleaning sequence has on the photosensitive body.
According to one aspect of the present invention, there is provided an image forming apparatus including a photosensitive body, a contact-transfer member that contacts said photosensitive body, an voltage applying circuit that applies voltage to said contact-transfer member, and a controller for causing the voltage applying circuit to apply intermediate voltage to the contact-transfer member in the course of a switch from the voltage used for a cleaning sequence to the voltage used for transferring. In the cleaning sequence, the voltage applying circuit applies voltage, opposite in polarity to that used for the transferring, to the contact-transfer member for causing residual toner and paper particles on the contact-transfer member to return to the photosensitive body. By applying the intermediate voltage to the contact-transfer member prior to application of the transfer voltage, the photosensitive body surface voltage does not change steeply. Therefore, voltage of the photosensitive body is not disturbed, and good quality of printing is insured.
The control unit may cause the voltage applying circuit to apply the intermediate voltage to the contact-transfer member while the photosensitive body rotates once or twice. This eliminates the influence that application of the transfer voltage after the cleaning sequence has on the photosensitive body.
The intermediate voltage may be 0V or greater than 0V. The intermediate voltage may be applied more than once. For instance, a first intermediate voltage of 0V is applied and subsequently a second intermediate voltage greater than that is applied.
Additional objects, benefits and advantages of the present invention will become apparent to those skilled in the art to which the invention pertains from the subsequent description of the embodiment(s) and the appended claims, taken in conjunction with the accompanying drawings.